Optical Phenomena in Gemstones: Asterism, Chatoyancy & Colour-Change

Some gems do more than glow — they perform. A star glides across a sapphire's dome; a single bright line opens and closes across a cat's-eye; an alexandrite turns from green to red as you walk from daylight into lamplight. These are optical phenomena, and they are among the most captivating effects in the gem world. Here is how each one works and what to look for.

In one line: optical phenomena are special light effects — asterism (the star), chatoyancy (the cat's-eye), colour-change, adularescence and more — created by a gem's internal structure, and the finest examples are prized precisely for how cleanly they perform.

The phenomena at a glance

Asterism — the star

A star sapphire or ruby is cut as a smooth dome (en cabochon) over a fine mesh of microscopic rutile needles aligned within the crystal. That "silk" reflects light into a star — typically six rays — that glides across the dome as the stone moves. The key point most buyers miss: the inclusions are not a flaw here, they are the feature. Without them, there is no star, which is also why fine star sapphires are usually left unheated (heat dissolves the silk).

"I have a stunning 8.07-carat, GIA-certified, unheated, colour-change star sapphire from Sri Lanka. It is a sublime gem — exceptionally rare and incredibly beautiful." — David Saad, Skyjems

Chatoyancy — the cat's-eye

The same idea as asterism, but with inclusions aligned in a single direction rather than a mesh: the result is one bright line of light across the dome — the "cat's-eye." The finest is cat's-eye chrysoberyl, where the eye is sharp, centred, and shows a "milk-and-honey" effect (one side of the line bright, the other shadowed). Look for a crisp, mobile line that closes to a slit under a single light.

Colour-change — the gem that changes its mind

The most theatrical phenomenon. A colour-change gem absorbs light differently across the spectrum, so it reads one colour in daylight (or fluorescent) and another under incandescent (warm) light. Alexandrite is the celebrated example — green to red, the rarest and most valuable change — and colour-change sapphire offers the effect more accessibly (often blue-violet to purple). The stronger and more complete the change, the higher the value.

Adularescence, labradorescence & play-of-colour

• Adularescence is the soft blue-to-white glow that floats across a fine moonstone — caused by light scattering between thin internal layers.
• Labradorescence is the flash of metallic blues, greens and golds in labradorite (the brightest is called spectrolite).
• Play-of-colour is the famous shifting fire of opal, caused by diffraction through a microscopic lattice of silica spheres.

What to look for

For any phenomenal gem, judge the phenomenon first: is it sharp, centred and mobile? A star or eye should track cleanly across the dome; a colour-change should be strong and complete, not a faint hint. Then judge the body — colour, transparency and cut. And as always, confirm what a laboratory says: for a star sapphire, that the asterism is natural and the stone unheated, not a diffusion-induced effect.

Inquire with the Curator to watch a star or colour-change move under the light in person, or browse the sapphire collection. Toronto: 416-366-3335.

Frequently asked questions

What are optical phenomena in gemstones? Special light effects created by a gem's internal structure — including asterism (the star), chatoyancy (the cat's-eye), colour-change, adularescence (moonstone's glow), labradorescence and opal's play-of-colour. The finest examples are prized for how cleanly the effect performs.

What causes the star in a star sapphire? Light reflecting off a fine mesh of oriented rutile "silk" needles inside the stone. Cut as a smooth cabochon, that structure produces a six-rayed star. The inclusions are the feature, not a flaw — which is why fine star sapphires are usually unheated.

What is chatoyancy (the cat's-eye effect)? A single bright band of light across a domed stone, caused by reflection off fine parallel inclusions. The finest is cat's-eye chrysoberyl, prized for a sharp, centred, mobile eye.

How does a colour-change gemstone work? It absorbs light differently across the spectrum, so it appears one colour in daylight and another under incandescent light. Alexandrite (green to red) is the celebrated example; colour-change sapphire offers the effect more accessibly.

Are inclusions bad in a phenomenal gemstone? No — for star and cat's-eye gems, the inclusions create the effect. A flawless stone would show no star or eye. The goal is well-oriented inclusions that produce a sharp, mobile phenomenon.